Quaternary faults of the central Rocky Mountains, Colorado: A new seismotectonic evaluation

Quaternary faults in the central Rocky Mountain of CO exhibit normal displacement, are generally parallel to the strike of pre-existing Larmide structures, and typically occur in the hanging walls of Laramide thrust faults. These observations are consistent with models in which Mesozoic thrust faults are being reactivated as normal faults in the contemporary extensional tectonic setting. To assess the seismogenic potential of these faults, the authors evaluated the recency of fault movement and style of deformation via aerial reconnaissance, interpretation of aerial photography and field mapping of selected sites. The 82-km-long Red Rocks-Climarron fault zone shows evidence of late Quaternary displacement and may be capable of producing an M[>=]6.75 earthquake based on its total fault length and inferred fault width. Earthquake hypocenters indicate that the thickness of the seismogence crust in CO is similar to much of the western US (ca. 15 km). In additional to tectonic deformation, numerous faults and lineaments have been identified in the Paradox Basin and along the southern Grand Hogback monocline that are active due to diapiric movement of halite. In particular, active deformation along the Grand Hogback is limited to portions of the structure underlain by a 3-km-deep Pennsylvania halite basin. Because Quaternary deformationmore » along and near these large Laramide structures is due to the movement of halite rather than deep-seated tectonism, the maximum size of a potential earthquake is limited by the down-dip width and lateral extent of fault planes within brittle rocks overlying the halite. The authors infer that the maximum depth of brittle faulting due to diapiric halite flow is 6 km, and the earthquakes larger than M 5 are unlikely to occur on faults associated with the Grand Hogback and salt anticlines of the Paradox Basin. The 1984 Carbondale earthquake swarm (M[<=]3.2) may have been the result of such faulting.« less